Ferroelectric metal oxide ceramic materials such as lead zirconate titanate (PZT) have been investigated for use in ferroelectric semiconductor memory devices. The ferroelectric material is located between two electrodes to form a ferroelectric capacitor for storage of information. Ferroelectric capacitor uses the hysteresis polarization characteristic of the ferroelectric material for storing information. The logic value stored in a ferroelectric memory cell depends on the polarization direction of the ferroelectric capacitor. To change the polarization direction of the capacitor, a voltage which is greater than the switching voltage (coercive voltage) needs to be applied across its electrodes. The polarization of the capacitor depends on the polarity of the voltage applied. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed, resulting in a non-volatile memory cell.
Referring to FIG. 1, a group 102 of memory cells 105 are shown. The memory cells, each with a transistor 130 coupled to a capacitor 140 in parallel, are coupled in series. Such series memory architectures are described in, for example, Takashima et al., xe2x80x9cHigh Density Chain Ferroelectric Random Access Memory (chain FRAM)xe2x80x9d, IEEE Jrnl. of Solid State Circuits, vol.33, pp.787-792, May 1998, which is herein incorporated by reference for all purposes. The gates of the cell transistors can be gate conductors which are coupled to or serve as wordlines. A selection transistor 138 is provided to selectively couple one end 109 of the group to a bitline 150. A plateline 180 is coupled to the other end 108 of the group. Numerous groups are interconnected via wordlines to form a memory block. Sense amplifiers are coupled to the bitlines to facilitate access to the memory cells.
FIG. 2 shows a cross-section of a conventional memory group 202. The transistors 230 of the memory cells 205 are formed on a substrate 210. Adjacent cell transistors shared a common diffusion region. The capacitors 240 of the memory group are arranged in pairs. The capacitors of a capacitor pair share a common bottom electrode 241. The bottom electrodes are coupled to the cell transistors via active area bottom electrode (AABE) plugs 285. The top electrode 242 of a capacitor from a capacitor pair is coupled to the top electrode of a capacitor of an adjacent pair and cell transistors. The top capacitor electrodes are coupled to the cell transistors via active area top electrode (AATE) plugs 286. Between the electrodes is a ferroelectric layer 243. A barrier layer 263, such as iridium, is located between the electrode and the AABE plug. At a first end 209 of the group is a selection transistor (not shown) having one diffusion region coupled to a bitline. The other diffusion region is a common diffusion region with the cell transistor on the end of the group. A plateline is coupled to a second end 208 of the group. A lower barrier layer 289 is located between the gate and capacitor to protect the gate stack and contact from oxidizing during high temperature processes, such as an oxygen recovery anneal.
The series architecture theoretically enables a 4F2 cell size, where F is the feature size. However, conventional series architectures require a sufficient capacitance to produce a sufficient read signal for sensing. To produce the necessary capacitance, a capacitor with relatively large surface area is needed. This undesirably increases the cell size to greater than 4F2.
From the foregoing discussion, it is desirable to provide a memory group which avoids the disadvantages of conventional series memory architectures.
The invention relates generally to ICs. More particularly, the invention relates to ICs with a plurality of memory cells which having a series architecture. A memory cell of a series group comprises a transistor having a gate and first and second diffusion regions and a capacitor coupled to the transistor in parallel. In one embodiment, the capacitor includes first, second, and third electrodes separated by first and second dielectric layer to form first and second subcapacitors. The second electrode is a common electrode of the first and second subcapacitors. One of the first, second or third electrodes is coupled to the second diffusion region and the other two of the first, second, or third electrodes are coupled to the first diffusion region. In one embodiment, the memory cells are ferroelectric memory cells. By providing a capacitor of memory cell with two subcapacitors, one stacked on top of the other, the capacitance of the cell capacitor can be advantageously increased without increasing the surface area.